function curring2 (fn) {
    const that = this
    return function inner(...args) {
      const length = args.length;
      if(length !== 0 && args[length-1] == '+'){
        return fn.apply(that, args.slice(0, length-1))
      }
      return (...argus) => inner.call(that, ...argus, ...args)
    }
  }
  const calculate2 = curring2(function(...args){
      return args.reduce((acc, cur)=>acc+cur,0)
  })
  
  
  // 执行函数验证 
  console.log(calculate2(1,2)(4)(3,'+')) // --- 10